All over eastern Australia farmers are struggling to cope with great swarms of locusts. But where do these plagues come from?

The locust: A pest all over the world

The insect swarms could not have come at a worse time for the farmers, who were only just recovering from bushfires and the worst drought to hit the country in 100 years.

Unfortunately, it is precisely the conditions that were helping them to recover - a little rain, the growth of new shoots - that have also created the locust problem.

"The rain causes the crops to grow and the grass to grow - there's the [locusts'] food," Philip Blades of Blades Biological, a company supplying insects for education and research, explained to BBC World Service's Outlook programme.

"Then the rain produces damp soil to lay their egg tubes in... that has to be moist otherwise they'll dehydrate.

"So damp, growth of food, then they'll bloom. Once the food's gone they'll have to move on."

Biblical horror

This need to move on is what triggers the swarms.

Every day they take off at about 11 in the morning and fly 30-50km (20-30 miles).

They came in literally like a black haze

Australian farmer Phil Thompson

The horror of the plagues is nothing new. In the Bible, Exodus describes how God sent the locusts to punish Egypt: "When it was morning, the east wind brought the locusts.

"And the locusts went up over all the land of Egypt, and rested in all the coasts of Egypt: very grievous were they; for they covered the face of the whole Earth, so that the land was darkened."

Modern-day plagues are no less frightening.

"They came in literally like a black haze at about 12 o'clock, and by three or four in the afternoon they were everywhere, eating the crops out," New South Wales farmer Phil Thompson told Outlook.

"By the following morning the crops were gone."

Mr Thompson said he had been set to harvest 140 hectares (350 acres) of oats. The swarm has now put him back 12 months.

Mass flight

Aware of the potential problem, the Australian Plague Locusts Commission had sprayed around 185,000 hectares (460,000 acres) of land in the region with the aim of killing the immature locusts before they were able to fly.

The locusts are attracted to new growth

"I guess if they hadn't done that it would have been a lot worse," Mr Thompson added.

"But we didn't know the extent of the locusts left. Once they landed there was nothing we could do.

"And the worst is yet to come, because these guys we've got now have actually got to lay their eggs. In the spring, if we get a crop it'll be looking nice and green, and the locusts will hatch out again."

There can be up to 40 billion locusts in one swarm.

During one plague in Somalia, the locusts devoured enough food to feed 400,000 people for a whole year. The swarm covered 1,000 square km.

Again and again

"Locusts can be seen as a black cloud up to 30 miles away, 500 to 600 feet above ground level," said Captain AS Aralleh, who works for the Desert Locusts Control Organisation in north-east Africa.

"When you go through it, you can literally see nothing on the windscreen of the aircraft."

Locust swarms are currently on the move in southern Algeria and Mauritania. As in Australia, the main way they are being combated is through aerial spraying.

But the impact of this is still likely to be limited against the sheer numbers involved. A female locust can lay between 60 and 100 eggs, and be ready to spawn again within in a week.

In total, locusts can live for around 30 days. About 90% will die at the end of migration, although they will try to spawn first.

However, this means the locusts do form an extensive part of the food chain in some areas of the world. Some are eaten by human beings, but more usual predators include birds, reptiles and mammals.